Multimode fiber optic amplifier and method of amplifying optical signals

ABSTRACT

A multimode fiber receiver includes a multimode collimator, a multimode fiber optic amplifier and a detector. The collimator receives incoming signals and provides the signals to the amplifier. The amplifier includes plural amplification stages, a limiter, a tunable narrow band filter and a microcontroller. The amplification stages each include a gain element and a noise filter. The limiter receives the amplified signals and limits the energy of those signals. The optical signals subsequently traverse the narrow band filter including an adjustable pass band to provide desired signals to the detector. The microcontroller measures the energy of the incoming and output signals to control the limiter, amplification stages and/or narrow band filter in order to produce signals within the dynamic range of a particular application. The multimode fiber optic amplifier and/or receiver of the present invention is preferably utilized within an optical communication unit.

GOVERNMENT LICENSE RIGHTS

The U.S. Government may have a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.F33615-01-D-1849.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention pertains to optical communication systems. Inparticular, the present invention pertains to a multimode fiber opticamplifier employing dynamic gain adjustment to produce amplified opticalsignals with reduced noise and at desired levels to prevent saturationof an optical detector. The present invention amplifier is preferablyutilized within a receiving unit for an optical communications system.

2. Discussion of Related Art

Optical communication systems transmit information in the form ofoptical signals through the environment between optical communicationunits. The transmitted signals typically encounter atmospheric and otherconditions. An optical communication unit generally employs a largeoptic to focus received optical signals directly onto a detector forsignal detection. However, some communications units may utilize anamplifier to amplify received optical signals and enable detection ofweaker signals.

Optical amplifiers for amplifying optical signals within opticalcommunication systems have generally been implemented based on singlemode fibers. These types of amplifiers tend to have reduced noise.Basically, an optical fiber is typically cylindrical and includes acentral portion or core surrounded by an optical material or cladding.Light or optical signals are guided by the fiber through the core, whilethe cladding maintains the light within the core by internal reflection.Single mode fibers have a core with small dimensions, thereby enablinglight to traverse the core in a single ray. In contrast, multimodefibers include a core with greater dimensions enabling light to traversethe core in a plurality of rays or modes.

However, single mode amplifiers suffer from several disadvantages.Initially, the probability of reception of a transmitted optical signalat an optical communication unit is enhanced in accordance with thediameter and field of view (e.g., commonly referred to as the numericalaperture or acceptance angle) of an optical fiber receiving thetransmitted signal. These fiber characteristics should be maximized fora particular application to achieve increased reception probability.Since single mode fibers generally have a relatively small diameter anda standard numerical aperture, the probability of reception of atransmitted signal via a single mode fiber is limited. Further, singlemode fibers require complex connections relative to multimode fibers,thereby complicating the amplifier or system.

Although multimode fibers include a greater diameter to enhancereception probability, the additional modes provided by this type offiber produce noise (e.g., Amplified Spontaneous Emission (ASE)) thatmay dominate the desired single mode signal.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to amplify opticalsignals within a receiver of an optical communications unit via amultimode fiber optic amplifier producing amplified signals with reducednoise.

It is another object of the present invention to employ a tunable narrowband filter within a multimode fiber optic amplifier to provideamplified optical signals with reduced noise.

Yet another object of the present invention is to employ a dynamiclimiter within a multimode fiber optic amplifier to prevent amplifiedsignals from saturating an optical detector of an optical communicationsunit.

Still another object of the present invention is to dynamically controlamplification of optical signals by a multimode fiber optic amplifier toproduce signals within a desired range for detection by an opticaldetector of an optical communications unit.

The aforesaid objects may be achieved individually and/or incombination, and it is not intended that the present invention beconstrued as requiring two or more of the objects to be combined unlessexpressly required by the claims attached hereto.

According to the present invention, a multimode fiber receiver includesa multimode collimator, a multimode fiber optic amplifier and adetector. The collimator receives incoming signals and provides thesignals to the amplifier. The amplifier includes plural amplificationstages, a limiter, a tunable narrow band filter and a microcontroller.The amplification stages are arranged in a serial fashion and eachinclude a gain element and a noise filter, where the noise filterremoves noise from the amplified signals. The collimator signals areconveyed by multimode fiber to the amplification stages for processing.The gain applied by the amplification stages to the signals iscontrolled by the microcontroller. The limiter receives the amplifiedsignals from the amplification stages and limits the energy of theoptical signals in accordance with control signals from themicrocontroller. The optical signals from the limiter traverse thenarrow band filter to provide desired signals to the detector. The passband of the narrow band filter is adjusted in accordance with controlsignals from the microcontroller. The microcontroller measures theenergy of the incoming and amplifier output signals to provide theappropriate control signals and enable the multimode fiber opticamplifier to produce signals within the dynamic range of a particularapplication. The multimode fiber optic amplifier and/or receiver of thepresent invention is preferably utilized within an optical communicationunit, but may be utilized for any application to receive and/or amplifyoptical signals.

The present invention provides several advantages. In particular, thepresent invention amplifier is compatible with multimode fibers withmaximum dimensions of approximately fifty microns and other multimodecomponents (e.g., multimode collimator, etc.). Further, the presentinvention produces amplified signals with reduced noise by employing atunable narrow band filter to reduce signal noise levels. In addition,the present invention prevents saturation of optical detectors byproducing signals below a detector saturation level via a dynamiclimiter.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of specific embodiments thereof,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is block diagram of an exemplary optical communications systememploying an optical receiver with a multimode fiber optic amplifieraccording to the present invention.

FIG. 2 is a schematic block diagram of the receiver of FIG. 1 includingthe multimode fiber optic amplifier according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary optical communications system employing an optical receiverwith a multimode fiber optic amplifier according to the presentinvention is illustrated in FIG. 1. Specifically, optical communicationssystem 10 includes a plurality of optical transceivers 20 each locatedat a different site. The optical transceivers communicate with eachother based on transmission and reception of optical signals, preferablyin the form of laser signals. The exemplary system may be utilized forair-to-air or air-to-ground applications.

Transceivers 20 each transmit and receive optical signals and include areceive/transmit structure 22, an optics unit 24, an optical transmitter26, a position and tracking unit 28 and an optical receiver 30. Receiver30 includes a multimode fiber optic amplifier 50 according to thepresent invention. Structure 22 interfaces a surrounding environment toreceive and transmit optical signals, while optics unit 24 is coupled totransceiver components (e.g., optical transmitter 26, position unit 28and optical receiver 30) to direct signals to and from structure 22. Byway of example, structure 22 may be implemented by a conventional commonaperture telescope.

Optical transmitter 26 produces optical signals for transmission bystructure 22, while position and tracking unit 28 receives incomingoptical signals from optics unit 24 and other information (e.g., GPSlocation, etc.) and determines the appropriate settings to enable theoptical transceiver to lock onto an optical signal transmitted byanother optical transceiver 20.

Optical receiver 30 receives and processes incoming optical signals fromoptics unit 24. The optical receiver includes multimode fiber opticamplifier 50 to amplify optical signals received by the receiver fordetection and/or processing. This amplification enables the receiver todetect and/or process weak optical signals and provides the receiverwith enhanced sensitivity.

Optical receiver 30 including multimode fiber optic amplifier 50according to the present invention is illustrated in FIG. 2.Specifically, optical receiver 30 includes a multimode fiber collimator21, multimode fiber optic amplifier 50 and a detector 23. The collimatorand detector are typically implemented by conventional opticalcomponents and are preferably compatible with fifty micron multimodeoptical fiber. By way of example only, collimator 21 includes anumerical aperture of 0.14 and may accommodate a data stream of 3.125Gigabits per second (Gb/s).

Collimator 21 receives incoming optical signals from optics unit 24(FIG. 1). The collimator produces a collimated beam and is coupled to aconventional SMA type fiber connector 62 of amplifier 50 via a multimodefiber 51. Connector 62 is further coupled to a multimode fiber 53extending through amplifier 50 to convey the optical signals. Fibers 51,53 typically include transverse cross-sectional dimensions of fiftymicrons for compatibility with each other and the collimator. By way ofexample only, fiber 53 further includes a numerical aperture of 0.13.The transverse cross-sectional dimensions of fibers 51, 53 (e.g., fiftymicrons) provide a greater surface area to enhance collection efficiencyof the optical signal (e.g., by greater than 100 times relative to theefficiency of a single mode fiber) from the collimator. In addition, themultimode fibers require a less complex connection arrangement relativeto single mode fibers as described above.

Amplifier 50 includes amplification stages 80, 90, a limiter or switch52, a tunable narrow band filter 54 and a microcontroller 60.Amplification stage 80 receives and amplifies the collimated opticalsignals from fiber 53. The amplification stage includes a multimode gainelement or amplifier 82 and a noise filter 84. Gain element 82 ispreferably implemented by a multimode erbium doped fiber amplifier(EDFA). This type of amplifier generally includes a fiber doped witherbium or other rare earth element including atomic structures foramplifying light. Basically, energy is injected into the doped fiber tostimulate the atoms of the rare earth element to release stored energyin the form of light within a particular wavelength range (e.g., 1310nanometers or 1550 nanometers). A weak optical signal within thewavelength range (e.g., 1310 nanometers or 1550 nanometers) of thereleased energy and entering the fiber absorbs the released energyduring traversal of the fiber, thereby producing an amplified signal.Fiber optic amplifier 50 includes a pump laser 70 to inject theappropriate energy to stimulate the doped fiber and amplify the opticalsignals. The pump laser is typically coupled to fiber 53 to inject thestimulation energy into that fiber toward the input of gain element 82.By way of example only, the pump laser is implemented by laser diodesproviding laser signals including a wavelength of approximately 980nanometers. However, the pump laser may be implemented by anyconventional or other device providing laser or other energy signals ofany suitable wavelengths compatible with a doped fiber. The pump laseris controlled by microcontroller 60 to control stimulation of the dopedfiber and the gain applied to the optical signals.

The amplified signals from multimode gain element 82 are applied tonoise filter 84. The noise filter is preferably in the form of a fixedAmplified Spontaneous Emission (ASE) filter. The amplified signalsbasically contain the desired band and extraneous signals in the form ofwhite light. Noise filter 84 removes or filters the white light toprovide an amplified signal with reduced noise.

Amplified signals from amplification stage 80 (e.g., noise filter 84)are received by amplification stage 90 via fiber 53. Amplification stage90 is substantially similar to amplification stage 80 described aboveand includes a multimode gain element or amplifier 92 and a noise filter94. Gain element 92 is substantially similar to gain element 82 and ispreferably implemented by a multimode erbium doped fiber amplifier(EDFA) as described above. Pump laser 70 is further coupled to fiber 53toward the input of gain element 92 to inject the stimulation energy forthe gain element into that fiber. The pump laser is controlled bymicrocontroller 60 to control stimulation of the doped fiber of gainelement 92 and the gain applied to the optical signals as describedabove. The amplified signals from multimode gain element 92 are appliedto noise filter 94. The noise filter is substantially similar to noisefilter 84 and is preferably in the form of a fixed Amplified SpontaneousEmission (ASE) filter as described above. Noise filter 94 removes orfilters extraneous white light to provide an amplified signal withreduced noise as described above. The amplification stages basicallyenhance the sensitivity of receiver 30. By way of example only, receiver30 may be configured to include a sensitivity of −42 dBm.

Limiter 52 is preferably in the form of a conventional Variable OpticalAttenuator (VOA) and receives the amplified optical signals fromamplification stage 90 (e.g., noise filter 94) via fiber 53. The limitercontrols the energy of the optical signals to prevent saturation ofdetector 23 in accordance with control signals from microcontroller 60.By way of example only, the limiter attenuates signals exceeding 7.5 dBmto prevent saturation of the detector. However, the limiter mayattenuate signals of any desired signal level.

Narrow band filter 54 may be implemented by any conventional or otheroptical band pass filter. The narrow band filter receives the opticalsignals from limiter 52 via fiber 53 and filters those signals toprovide optical signals within a desired band or range. This furtherreduces noise and assists the receiver with respect to locking onto atransmitted signal. The pass band of filter 54 is adjustable bymicrocontroller 60 to provide signals over a desired dynamic range. Byway of example only, the filter may limit the optical band within a onenanometer range.

Microcontroller 60 may be implemented by any conventional or othermicroprocessor, controller or circuitry. The microcontroller controlsamplifier 50 to optimize the amplified output signal and performsvarious safety monitoring to ensure proper receiver operation. Inparticular, the microcontroller provides control signals to limiter 52,narrow band filter 54 and pump laser 70 to control the gain of theoptical signals. Specifically, an input photodiode 66 is disposed alongfiber 53 between connector 62 and amplification stage 80 (e.g., gainelement 82). The input photodiode may be implemented by any conventionalor other optical sensor to measure the strength of the incoming opticalsignal. Similarly, an output photodiode 68 is disposed along fiber 53subsequent narrow band filter 54. The output photodiode may beimplemented by any conventional or other optical sensor to measure thestrength of the amplified optical signal. Photodiodes 66, 68 are coupledto microcontroller 60 to provide optical signal measurements. Themicrocontroller processes the signal measurements and other informationto control attenuation of limiter 52, the pass band of narrow bandfilter 54 and the gain of amplification stages 80, 90 (via pump laser70) to produce signals in the desired dynamic range, preferably belowthe saturation level for detector 23. For example, when the outputsignal or gain (e.g., the amplified output signal divided by theincoming optical signal) attains a level exceeding a desired range, themicrocontroller may lower the gain of the amplification stages, increasethe attenuation of limiter 52 and/or adjust the pass band of the narrowband filter. If the output signal or gain attains a level below adesired range, the microcontroller may increase the gain of theamplification stages, decrease the attenuation of limiter 52 and/oradjust the pass band of the narrow band filter. The microcontrollerpreferably employs conventional techniques to process the signalmeasurements and determine the parameters to control the amplifiercomponents. The microcontroller and photodiodes basically form afeedback loop to provide a variable gain and produce desired opticalsignals.

The microcontroller further monitors unit or system conditions and mayperform various actions in response to those conditions. Specifically,the microcontroller may interface a processing system or other device(e.g., sensor, etc.) of the optical receiver or transceiver via anyconventional or other suitable interface (e.g., RS-232, RS-422, etc.) toreceive and/or provide information (e.g., temperature, TTL signals,alarms, etc.). The microcontroller processes the information todetermine system conditions and may control various system components.For example, the microcontroller may receive temperature signals anddisable the system (or specific system components) and/or provide alarmsin response to excessive temperatures. Further, the microcontroller maybe coupled to temperature devices (e.g., thermoelectric or othercooling/heating devices, etc.) and control those devices to maintain adesired operating temperature or temperature range for the system orunit.

The resulting signals from narrow band filter 54 are provided tomultimode fiber 53. The multimode fiber is coupled to a conventional SMAtype connector 64 of amplifier 50. Detector 23 is coupled to connector64 via a multimode fiber 57 to receive the amplified signals fordetection and/or processing by optical receiver 30. Fiber 57 typicallyincludes transverse cross-sectional dimensions of fifty microns forcompatibility with fiber 53 and the detector. The transversecross-sectional dimensions of fiber 57 (e.g., fifty microns) match thedimensions of the detector to minimize or prevent deficient coupling andleakage, thereby maximizing the optical signals received by thedetector. The multimode fiber further requires a less complex connectionarrangement relative to single mode fibers as described above. Thedetector preferably operates at a bandwidth in the approximate range oftwo to three gigahertz, and provides a complementary match withmultimode fibers 53, 57. By way of example, the detector may beimplemented by a PIN type detector coupled to a fifty micron fiber(e.g., fiber 57).

Operation of the present invention amplifier is described with referenceto FIG. 2. Specifically, fiber 53 receives incoming optical signals andconveys those signals to amplification stage 80. The incoming opticalsignals may be provided by various sources depending upon a particularapplication. By way of example only, the optical signals are provided bymultimode collimator 21 of optical receiver 30 that is coupled tomultimode fiber 53 via multimode fiber 51 and connector 62 as describedabove. Amplification stage 80 amplifies and filters noise from theincoming optical signals, where the amplified signals are furtheramplified and noise filtered by amplification stage 90. Theamplification stages are controlled by microcontroller 60 as describedabove. Limiter 52 receives the amplified signals from amplificationstage 90 and attenuates the optical signals in accordance with controlsignals from microcontroller 60 as described above. Narrow band filter54 receives the attenuated signals and provides optical signals within adesired pass band in accordance with control signals frommicrocontroller 60 as described above. The microcontroller controls theamplifier components in accordance with the strengths of the input andamplified optical signals measured by photodiodes 66, 68 as describedabove. The resulting amplified signals are provided to multimode fiber53 and may be supplied to various devices depending upon a particularapplication. By way of example only, the resulting amplified signals areprovided to detector 23 of optical receiver 30 that is coupled tomultimode fiber 53 via multimode fiber 57 and connector 64 as describedabove.

It will be appreciated that the embodiments described above andillustrated in the drawings represent only a few of the many ways ofimplementing a multimode fiber optic amplifier and method of amplifyingoptical signals.

The present invention amplifier may amplify any types of optical signals(e.g., light, laser, etc.) of any suitable wavelength or frequency. Thepresent invention amplifier may receive optical signals from and provideamplified optical signals to any suitable optical devices (e.g.,collimator, detector, single mode, multimode, etc.). The presentinvention optical receiver may receive any types of optical signals(e.g., light, laser, etc.) of any suitable wavelength or frequency. Thereceiver may be utilized within any suitable device accommodatingoptical signals (e.g., optical transceiver, optical communicationssystem or unit, etc.). The receiver and amplifier may accommodateoptical signals at any desired data rate (e.g., gigabits, megabits,etc.).

The collimator may be implemented by any quantity of any conventional orother optical device to provide collimated optical signals (e.g.,multimode, single mode, etc.). The detector may be implemented by anyquantity of any conventional or other detection devices to detectoptical signals.

The present invention amplifier may include any quantity of components(e.g., amplification stages, gain elements, filters, limiters, etc.)arranged in any fashion. The limiter may be of any quantity, may beimplemented by any conventional or other optical attenuation device(e.g., Variable Optical Attenuator, etc.) and may attenuate the opticalsignal by any desired amount. The limiter may have a fixed orpredetermined attenuation, or the attenuation may be adjustable ordynamic. The narrow band filter may be of any quantity, may beimplemented by any conventional or other optical filter and may providesignals within any desired pass band (e.g., optical signals of anydesired wavelength or within any desired wavelength range, etc.). Thenarrow band filter may have a fixed or predetermined pass band, or thepass band may be adjustable or dynamic.

The amplifier may include any quantity of amplification stages arrangedin any fashion (e.g., serial, parallel, separated by any quantity ofoptical devices, etc.). The amplification stages may include anyquantity of gain elements, filters or other optical devices arranged inany fashion. The gain elements may be of any quantity, may beimplemented by any conventional or other optical amplifier or gainelement and may apply any desired gain (e.g., amplification,attenuation, etc.) to provide any desired signals. The gain elements mayhave a fixed or predetermined gain (or attenuation), or the gain (orattenuation) may be adjustable or dynamic. The gain elements may employany desired rare earth element (e.g., erbium, etc.) and may bestimulated by any quantity of any desired energy source (e.g., pump orother types of lasers or light sources, etc.) providing any type ofenergy to any quantity of gain elements at any desired wavelengths orfrequencies. Each energy source may provide stimulation energy to anyquantity of gain elements. The noise filters may be of any quantity, maybe implemented by any conventional or other optical filter (e.g., bandpass, high pass, etc.) and may pass or reject any desired band to removeany type of noise (e.g., ASE, etc.) or other extraneous signals from theamplified signals. The noise filters may have a fixed or predeterminedconfiguration for specific extraneous signals (e.g., a fixed orpredetermined pass band), or the noise filter may be adjustable ordynamic.

The input and output photodiodes may be implemented by any quantity ofany conventional or other optical sensors to measure the strength ofoptical signals. The amplifier may include any quantity of any type ofoptical sensors disposed at any locations along or external of theamplifier signal path to measure any characteristics (e.g., signalstrength, etc.).

The microcontroller of the present invention amplifier may beimplemented by any conventional or other microprocessor, controller orcircuitry to perform the functions described herein. Alternatively, anyquantity of processors or processing devices or circuitry may beemployed within the present invention amplifier, where the processorfunctions may be distributed in any fashion among any quantity ofhardware and/or software modules, processors or other processing devicesor circuits. The microcontroller may generate any types of controlsignals of any format to control any quantity of the amplifiercomponents (e.g., limiter, narrow band filter, gain elements/pump laser,etc.). The microcontroller may control the amplifier components based onany desired information (e.g., optical signal strength, information fromthe optical receiver or optical transceiver, etc.) and may interface anyprocessing system via any suitable interface (e.g., RS-232, RS-422,etc.) to transfer any desired information (e.g., alarms, temperature,measurements, conditions, etc.). The microcontroller may further becoupled to any sensing or other devices (e.g., sensors, processors,circuitry, etc.) to monitor any desired system conditions (e.g.,temperature, power, etc.) and control any suitable devices (e.g.,heater/cooling units, power unit, alarms, etc.) in response to thoseconditions. For example, the microcontroller may indicate an alarm,disable power or control heating/cooling units in response to atemperature condition.

The software for the microcontroller of the present invention amplifiermay be implemented in any suitable computer language, and could bedeveloped by one of ordinary skill in the computer and/or programmingarts based on the functional description contained herein and thefigures illustrated in the drawings. Further, any references herein ofsoftware performing various functions generally refer to processorsperforming those functions under software control. The algorithmsdescribed above may be modified in any manner that accomplishes thefunctions described herein.

The optical signals may be conveyed via any quantity of any suitablecarriers (e.g., multimode fiber optics, etc.) of any type, size orshape. The carriers may include any desired characteristics (e.g.,numerical aperture, etc.). The carriers may be configured for anydesired data rate (e.g., gigabits, megabits, etc.). The connectors maybe implemented by any quantity of any conventional or other opticalconnectors (e.g., SMA, etc.) and may be disposed at any suitablelocations within or external of the amplifier or receiver.

The present invention receiver and amplifier are not limited to theapplications disclosed herein (e.g., optical communication systems), butmay be utilized for any application employing reception and/oramplification (or attenuation) of optical signals.

From the foregoing description, it will be appreciated that theinvention makes available a novel multimode fiber optic amplifier andmethod of amplifying optical signals, wherein a multimode fiber opticamplifier employs dynamic gain adjustment to produce amplified opticalsignals with reduced noise and at desired levels to prevent opticaldetector saturation.

Having described preferred embodiments of a new and improved multimodefiber optic amplifier and method of amplifying optical signals, it isbelieved that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims.

1-9. (canceled)
 10. A method of amplifying optical signals comprising:(a) receiving an optical signal from an optical signal source on amultimode fiber forming a signal path; (b) measuring strength of saidoptical signal along said signal path; (c) amplifying said opticalsignal in a multimode format and filtering noise from said amplifiedoptical signal, wherein said amplification and filtering of saidmultimode optical signal include a plurality of amplification stagessuccessively coupled to each other, and wherein step (c) furtherincludes for each stage: (c.1) amplifying a multimode optical signalreceived by that stage; and (c.2) filtering noise from said opticalsignal amplified by that stage; (d) receiving and attenuating saidamplified optical signal from a final one of said amplification stagesto produce an attenuated signal; (e) filtering said attenuated signaland providing a resulting amplified signal within a desired band; and(f) measuring strength of said resulting amplified signal andcollectively controlling said amplification of said optical signal bysaid amplification stages, said attenuation of said amplified opticalsignal and said filtering of said attenuated signal to control the gainof said resulting amplified signal in accordance with at least saidmeasured signal strengths.
 11. (canceled)
 12. The method of claim 10,wherein step (c.1) further includes: (c.1.1) amplifying said multimodeoptical signal via a doped fiber amplifier.
 13. The method of claim 12,wherein step (c.1.1) further includes: (c.1.1.1) providing energy tostimulate a doped fiber within said doped fiber amplifier; and step (f)further includes: (f.1) controlling energy provided to said doped fiberin accordance with said measured signal strengths to control a gain ofsaid doped fiber amplifier.
 14. The method of claim 10, wherein saidmultimode fiber includes a transverse cross-sectional dimension lessthan or equal to fifty microns.
 15. The method of claim 10, wherein amultimode collimator is coupled to said multimode fiber to serve as saidsignal source, and step (a) further includes: (a.1) receiving acollimated optical signal on said multimode fiber from said multimodecollimator.
 16. The method of claim 15, wherein an optical detector iscoupled to said multimode fiber, and step (d) further includes: (d.1)attenuating said amplified optical signal to a level below a saturationlevel of said optical detector; and said method further includes: (g)detecting said resulting amplified signal via said detector.
 17. Themethod of claim 10, wherein said desired band is adjustable, and step(e) further includes: (e.1) controlling said filtering of saidattenuated signal to pass optical signals within a specified band. 18.The method of claim 10, wherein said optical signals are amplifiedwithin an optical communication unit of an optical communication system,and step (a) further includes: (a.1) receiving said optical signal inthe form of a transmitted optical signal from another opticalcommunication unit on said multimode fiber.